160 



BOTANY 



TART I 



growth in thickness of the stem, and has then the furrowed appear- 

 ance so characteristic of the majority of okl tree-trunks. The usual 

 brown or red colour of bark, just as in similarly coloured heart-wood, 



is occasioned by the presence of 





tannins, to the preservative 

 qualities of which is due the 

 great resistance of bark to the 

 action of destructive agencies. 



V 



The i)eculiar white colour of 

 Birch -bark is caused by the 

 presence of betulin (birch-resin) 

 in the cells. 



In roots which grow in thick- 

 ness the phellogen usually de- 

 velops in the pericycle (Fig. 

 160 i), Z'), and in consequence 

 of this the primary cortex of the 

 roots dies and peels off. The 

 succeeding phellogen layers are 

 ^L^L^T^f formed in exactly the same way 

 Lir^-=$^^ in the root as in the stem. 



Lentieels (^^^). — In most 

 woody plants, particularly in 

 Dicotyledons, cortical pores, or 

 LENTICELS (Fig. 167), make their 

 appearance simultaneously with 

 the formation of periderm. The 

 leriticels take their origin in a 

 phellogen layer (pi) which, in 

 the case of peripheral cork forma- 

 tion, almost always develops 

 directly under the stomata. The 

 phellogen, from which the len- 



FiG. 108. — Transverse section of the periplieial 

 tissues of the stem of Quercus sessiliflora. 1, 2, 3, . 



Successively formed hayers of cork; pr, primary ticcls arise. Unlike the COrk phel- 

 cortex, modilied by subsequent growtli ; in- 

 ternally to p', pericycle; sc, sclerencliyiuatous 

 fibres from the ruptured ring of sclerenchy- 

 niatous fibres of the pericycle ; s, subsequently of COMPLEMENTARY CELLS (/) 

 formed sclereides ; «', sclereides, of secondary with intercellular spaCCS between 

 ast fibres with accompanying crystal ., ^^ ,i • -i , 



logen, does not form cork cells, 

 but a lenticel tissue composed 



growth ; tr, bast tibres witli accompanying cry Sv«i , /\ i • • i i 



cells; k, cells with aggregate crystals. All the them. Ou the inSlde, howCVCr, 

 tissue external to the innermost layer of cork a phellodcrm is regularly derived 

 is dead and di.scolouredan.l has become trans- ^^.^^^ ^^ phellogen. The COm- 

 formed into bark, (x 225.) t' s> 



plementary cells press the epi- 

 dermis outwards and finally rupture it. Where the complementary 

 cells are only loosely united, intermediate bands or closing layers are 

 developed from the phellogen alternately with the layers of looser 

 tissue ; the closing layers become eventually ruptured. The comple- 



11 



